Help with sleep/lock button

Yesterday, I bought an iPhone 4 for my girlfriend from a person off of craigslist. When I looked at it everything seemed to be fine and I didn't notice a problem with it until later on. The problem is that it was randomly locking for no reason and when I push the lock/sleep button, it locks the screen but instead of fading black and sleeping, it goes to the lockscreen(with the clock/slide to unlock) and then asks me to slide to power off or cancel, it does this everytime I push the lock button. I looked online and alot of people gave others the solution of restoring and updating it so I restored it to factory settings and updated to 6.1.3 but it is still doing. I kept playing with it and it doesnt appear to randomly lock anymore or atleast I hope so, but it DOES keep doing the slide to power off thing. Also, the lock button is very sensitive and it doesnt take much of a tap for it to do all of this. I was just wondering if you guys would happen to either know if it is just the lock button that is broke and if I replaced the lock button it would fix it or maybe if there was a app, perhaps on Cydia, that could make it where the lock/sleep button just doesn't do anything, because i can use AssistiveTouch to lock the screen. Aside from the whole lock button screen the phone is in pretty great condition and works like a charm. I will appreciate any help you guys can provide!